1) Field of the Invention
This invention relates to an expanded thermoplastic resin product having excellent physical properties and surface appearance, and more specifically to an injection-expansion molded, thermoplastic resin product comprising an expanded portion, which contains cells of a very small average cell diameter and a uniform average cell population, and as a surface layer, an unexpanded portion integrated with the expanded portion and having a good external appearance. This invention is also concerned with a process for producing the expanded product by using carbon dioxide and/or nitrogen as a blowing agent.
2) Description of the Related Art
For the production of expanded thermoplastic products, processes making use of a chemical blowing agent or physical blowing agent are known.
A chemical expanding process generally comprises mixing raw material with an organic blowing agent of a low molecular weight, which decomposes at a molding temperature to produce gas, and then heating the resulting mixture to a decomposing temperature of the blowing agent or higher to effect expansion molding. According to this process, the production of gas is proportional to the molding temperature, and the decomposition temperature can be easily adjusted by adding an expanding aid or the like. Moreover, this process can obtain expanded products having closed cells.
However, in addition to high production cost for the use of a special blowing agent, these expanded products tend to develop discoloration, offensive odor, food sanitation problems and the like due to decomposition residues of the blowing agent, said decomposition residues remaining in the expanded products. There are other problems including smearing of molding machines caused by a chemical blowing agent and defective molding associated with such smearing.
On the other hand, a gas expanding process is a physical expanding process and comprises melting a resin in a molding machine, feeding an organic compound of a low boiling point such as butane, pentane or dichlorodifluoromethane to the resin, kneading the resin and the organic compound together, and then releasing the resulting mixture into a low-pressure zone to effect expansion molding. The organic compound of the low melting point, which is employed in this process, has high compatibility with the resin and is hence excellent in solubility and also in retention, so that it features the availability of expanded products of high expansion ratios. Nonetheless, such blowing agents are costly and moreover, have dangers such as inflammability and toxicity. They also have a potential problem of air pollution. Further, there is a move toward the total ban of freon series gases led by dichlorodifluoromethane in view of environmental problems such as destruction of the ozonosphere.
With a view to overcoming such problems of the conventional processes, numerous processes making use of an inert gas such as carbon dioxide gas or nitrogen as a blowing agent, said inert gas being clean and economical, have been proposed. However, the inert gas has poor solubility in a resin because of its low compatibility with the resin. Expanded products have large and uneven cell diameters and low cell populations, leading to problems in external appearance, mechanical strength, heat insulating properties and expansion ratio.
Concerning injection-expansion molding for structural foams, various processes have also been proposed. Expanded products available by these processes have stiffness 3 to 4 times as much as conventional injection-molded products, because a sandwich structure composed of surface skin layers and an intermediate core layer is formed and their volumes are effectively increased by the expansion when compared at the same resin weight. It has however been pointed out that cell diameters of expanded products produced by these processes are as large as 50 to 100 .mu.m and are uneven and hence that in impact strength tests, their cells become starting points for breakage to result in lowered impact strengths.
As a technique for resolving these problems, U.S. Pat. No. 4,473,665 discloses a production process for obtaining an expansion-molded product with very small cells of 2 to 25 .mu.m in diameter uniformly distributed therein. According to this process, a thermoplastic resin sheet is first impregnated under pressure with an inert gas until saturation. Thereafter, the sheet is heated to a glass transition temperature of the thermoplastic resin, and is then depressurized so that the gas impregnated in the resin is brought into an over-saturated state to form cell nuclei. The sheet is then rapidly cooled so that the growth of cells is controlled. Further, a production process by extrusion forming or injection molding is also exemplified. This expansion forming or injection molding makes use of a process which comprises heating and melting a thermoplastic resin, which has been saturated beforehand with an inert gas under pressure, shaping the thus-molten thermoplastic resin under pressure, cooling and depressurizing the resin to form cell nuclei, and then cooling the resin to control its cell diameter. By these processes, expanded products containing a number of very small cells can be obtained. It is however substantially difficult to industrially practice these processes, because an inert gas has low compatibility with a resin and more than ten hours are required to fully impregnate the resin with the gas.
U.S. Pat. No. 5,158,986 discloses a technique for obtaining an expanded product, which has extremely small cell diameters and a high cell population, by using a supercritical fluid as a blowing agent and impregnating a thermoplastic resin with the supercritical fluid. As a supercritical fluid has excellent solubility similar to that of a liquid and superb diffusibility close to that of a gas, the resin can be impregnated with the blowing fluid in a short time. Two processes are proposed for obtaining expanded products in this patent publication, one comprising forming a thermoplastic resin into a sheet through an extruder, introducing the sheet into a pressurized chamber filled with carbon dioxide in a supercritical state to impregnate the sheet with carbon dioxide, and then heating the sheet in an expanding chamber under atmospheric pressure to cause the sheet to expand; and the other comprising melting a resin in an extruder, impregnating the molten resin with carbon dioxide in a supercritical state, extruding the resulting impregnated resin into a sheet-like product, introducing the sheet-like product into a pressurized chamber to form cell nuclei due to the pressure difference, and then heating or cooling the resultant sheet to control the diameter and population of cells.
Both the processes however require large-scale high-pressure facilities and hence an enormous initial cost and are poor in work efficiency, so that they can be hardly practiced on an industrial scale. Further, the former process requires a long time for the full impregnation of the sheet-like product with carbon dioxide because the sheet-like product is directly impregnated. on the other hand, the latter process impregnates carbon dioxide into the molten resin so that the penetration speed of carbon dioxide in the latter process is faster than that in the former process. It is however difficult to perform both the mixing of carbon dioxide and the formation of numerous cell nuclei by kneading the molten resin and carbon dioxide in only one extruder, thereby making it difficult to obtain an expanded product having numerous very small cells.
The present inventors proposed in Japanese Patent Laid-Open No. 11190/1996 a process for producing an expanded thermoplastic resin product evenly containing numerous very small cells by expansion extrusion, characterized in that the process comprises a gas dissolving step of impregnating a molten thermoplastic resin with an inert gas as a blowing agent in a first extruder and an adapter connected to the first extruder and having a mixing portion, a cooling step of lowering the temperature of the molten resin in a second extruder while maintaining its pressurized state, a nucleus-producing step of producing numerous cell nuclei as a result of an abrupt drop in pressure, and an expansion controlling step of controlling the diameters of cells.
According to this production process, it is possible to continuously perform the production of expanded products whose production is practically extremely difficult by the production process disclosed in U.S. Pat. No. 4,473,665 or U.S. Pat. No. 5,158,986. It is however necessary for this production process to make the lip opening of a die extremely narrow to apply high shear force to molten resin during the nucleus-producing step. Accordingly, it is becoming increasingly clear that this production process is suitable for the production of a thin expanded product but is not adequate for the production of a relatively thick expanded product. It is to be noted that Japanese Patent Laid-Open No. 11190/1996 discloses only extrusion processes.
U.S. Pat. No. 5,158,986 also proposes a process for obtaining an expanded product by molding a thermoplastic resin in a cylinder of an injection molding machine, impregnating the thus-molten resin with supercritical carbon dioxide, and upon uniform dispersion of carbon dioxide, abruptly heating the thus-impregnated resin to form cell nuclei, and then injecting the molten resin into a mold which has been filled with a high-pressure gas to control expansion.
This process is however accompanied by drawbacks such as:
1) since the melting of the resin, the kneading of carbon dioxide and the injection are all conducted by the injection molding machine alone and further because metering of the resin is stopped upon injection, it is difficult to assure the supply of carbon dioxide, which is fed continuously, in a constant quantity and also to maintain constant the mixing ratio of the thermoplastic resin to carbon dioxide; and PA1 2) as the screw back-pressure is no longer applied subsequent to the completion of metering insofar as a conventional injection molding machine is singly used, the thermoplastic resin and carbon dioxide in a mutually-dissolved state are separated from each other, thereby making it difficult to form very fine cells when they are injected into the mold. PA1 (I) as a gas dissolving step, melting 100 parts by weight of a thermoplastic resin at 100 to 450.degree. C. in a continuous plasticator (1) equipped with a feed line for a blowing agent, adding supercritical carbon dioxide and/or nitrogen in a proportion of 0.1 to 30 parts by weight per 100 parts by weight of the thermoplastic resin, and forming a molten resin composition in which the thermoplastic resin and the supercritical carbon dioxide and/or nitrogen are in a mutually-dissolved state, PA1 (II) as a cooling step, cooling, within the continuous plasticator (1), the molten resin composition to a temperature of from 50 to 300.degree. C. while maintaining a pressure equal to or higher than a critical pressure of the carbon dioxide and/or nitrogen, PA1 (III) as a metering and injection step, metering the thus-cooled molten resin composition by an injector (7), which is connected to the continuous plasticator (1) and equipped with an injection plunger (6), and filling the same in a mold (8), and PA1 (IV) as an expansion controlling step, lowering an internal pressure of the mold (8) to a pressure lower than the critical pressure of the carbon dioxide and/or nitrogen to produce cell nuclei, whereby the cell diameter is controlled;
These drawbacks have therefore led to a problem such that cells of a uniform and very small diameter can hardly be obtained in the case of a large molded product although an expanded product of very small cells is available insofar as the expanded product is a small molded product like specimens for the measurement of physical properties.